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Figure 2.7 Stratigraphic case study: description and definition of the litho-, bio- and chronostratigraphy of the stratotype section of the Wenlock Series, along Wenlock Edge in Shropshire, UK. This is the internationally accepted standard for the Wenlock Epoch, the third time division of the Silurian Period.

Figure 2.7 Stratigraphic case study: description and definition of the litho-, bio- and chronostratigraphy of the stratotype section of the Wenlock Series, along Wenlock Edge in Shropshire, UK. This is the internationally accepted standard for the Wenlock Epoch, the third time division of the Silurian Period.

section must, of course, be satisfied. The golden spike, which represents a point in the rock section and an instant in geological time, is then driven into the section, at least in theory. In practice the spike is usually adjusted to coincide with the first appearance (FAD) of a distinctive, recognizable fossil within a well-documented lineage. The ranges of all fossils occurring across the boundary are documented in detail as aids to correlating within the section and with sections elsewhere. Establishing stratotypes and golden spikes requires international agreement, and that can sometimes be hard to achieve (Box 2.3)! This horizon will then be the global standard section and point (GSSP) for this stratotype.

The Wenlock Epoch (time) was one of the first intervals of geological time to be defined with reference to a stratotype section for the Wenlock Series (rock) (Fig. 2.7). A lithostratig-

raphy was first established in the historic type area by definition of formations and members. On the basis of detailed collecting through the stratigraphy a succession of biozones was then defined, based on the ranges of characteristic graptolite faunas. Finally, a succession of stages was established together with two chr onozones. This remains the international yardstick for Wenlock time. When discussing geological time, we generally use the adjectives early, mid and late, but when dealing with rock the use of lower, middle and upper is more appropriate.

Sequence stratigraphy: using transgressions and regressions_

North American oil geologists developed a whole new system in the 1960s called sequence stratigraphy, an approach that emphasizes the importance of unconformities. In the early 1960s Larry Sloss recognized that the Pha-nerozoic rocks of the old North American continent could be split into six main cycles separated by unconformities (Fig. 2.9). These were large-scale cycles describing the major changes in sea level across an entire continent and through over 500 myr of Earth history. More minor sequences could be recognized within these major cycles. The fact that sedimentary rocks can be described as packets of strata, presumably deposited during trans-gressive events (when the sea floods the land), divided by periods of non-deposition during regressions (when the sea withdraws from the land), forms the basis for sequence stratigraphy.

The Ordovician System was born out of controversy, with Charles Lapworth taking the disputed overlapping strata between Sedgwick's Cambrian System and Murchison's Silurian System (see above). Despite the best efforts of British specialists (e.g. Fortey et al. 1995), they and many other international experts have pointed out that - although the classic British series and stages have wide global usage - they were based largely on endemic shelly faunas with only local and regional distributions, some units are bounded by disconformities (minor gaps in deposition, where the rocks below and above are oriented similarly, in contrast to the larger chunks of time represented by unconformities), and some have significant overlaps with adjacent series. Moreover many of the key sections are located in poorly exposed sections. In order to assemble a consolidated chronostratig-raphy that would work internationally, definitions in new sections were necessary.

First, it was decided in the 1980s by the International Subcommission on Ordovician Stratigraphy, a group of highly-qualified experts drawn from all over the world, that basal stratotypes for chro-nostratigraphic units should be correlated by means of conodonts and graptolites, the most effective of all Paleozoic zone fossils. Second, there should only be three series, defined as lower, middle and upper; and third, new sections must be sought to define a new set of global stages: the first was ratified in 1987, and the last in 2007. This has not been without rancour. Colleagues from around the world have clashed noisily at meetings defending their "own" sections, and sometimes national pride and access to further research funding have influenced voting. Nevertheless, a consensus is emerging and all the new stages are defined and in place, based on diverse sections such as a road section and river bank in South China (Hirnantian) and the coast of western Newfoundland (Tremadocian). Some older names such as Hirnantian and Tremadocian have been retained with slightly different definitions, whilst some are new, such as Floian and Sandbian, both based on stratotype localities in Sweden. This new structure is already providing a much more accurate time framework to describe, analyze and model Ordovician Earth systems (Fig. 2.8).